1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus which provides a stereoscopic image observable three-dimensionally.
2. Description of Related Art
Conventionally, various attempts have been made to reproduce and display a three-dimensional image. Of the attempts, a widely used approach is to utilize binocular parallax to provide an observer with stereoscopic vision. However, a parallax image provided for an observer in the typical approach using binocular parallax is a two-dimensional image. When such a two-dimensional image is observed, a focal point for an observed object adjusted in the eyeballs of an observer is fixed to the two-dimensional image plane, so that a contradiction arises between stereoscopic recognition by the function of adjusting the focal point in the eyeballs and stereoscopic recognition by the binocular parallax, and thus the observer often feels fatigued and irritated. For this reason, several approaches have been studied which do not rely only on the binocular parallax but make use of the stereoscopic recognition function provided by the eyeballs.
As one of those methods, Telecommunications Advancement Organization of Japan issued “Final Outcome Reports of Advanced Stereoscopic Motion Picture Communication Project” in 1997. In Section 8 of Chapter 3 of this document, “Studies of Stereoscopic Vision of Super-Multiview Regions” describes that, in displaying a stereoscopic image of “super-multiview regions” for displaying parallax images from a multiplicity of viewpoints corresponding to small angle increments in parallax such that a plurality of parallax images are incident on the pupil of a single eye of an observer, a focal point adjusted by the eyes of the observer is brought near a pseudo-stereoscopic image induced by binocular parallax to reduce fatigue and irritation of the observer.
In other words, the Report remarks that stereoscopic display with less fatigue of the eyes can be achieved through “a monocular parallax effect” when the conventional method of displaying stereoscopic images captured on two viewpoints to the two eyes of an observer is extended to a method of presenting parallax images captured on n viewpoints to the single eye of an observer keeping a distance between two adjacent points of the n viewpoints is set to be smaller than the pupil of the observer.
In addition, “Research and Development of Multiview Stereoscopic Display with Focused Light Array (FLA)” in Section 6 of Chapter 3 of the Report shows a specific example for putting the theory into practice. FIG. 52 shows the structure of the specific example. In FIG. 52, reference numeral 101 shows an FLA which has the structure as shown in FIGS. 53(A) and 53(B).
The FLA 101 has a plurality of units, each of which is shown in FIG. 53(A). In each unit, light from a light source 101a such as a semiconductor laser is shaped into thin luminous flux by an optical system 102. Such units are arranged in arc form as shown in FIG. 53(B) to constitute the FLA 101 which converges all luminous flux to the center (a focal point FP) of the circle.
As shown in FIG. 52, the focal point FP thus formed is again formed into an image on a vertical diffuser 106 through optical systems 102 and 105, and two-dimensional scanning is performed at high speed by scanning systems 103 and 104 to form a two-dimensional image 108. When the scanning is performed in a cycle shorter than the permissible time of persistence of vision (after image) of an observer 107 (equal to or smaller than approximately 1/50 seconds), the image can be observed without flicker.
It is presumed that a focal point at an instant forms each pixel of the two-dimensional image, and the respective pixels serve as bright points from which light rays emerge in different directions corresponding to the number of the light sources. The direction in which a light ray emerges can be determined by selecting which light source emits light. Since the light ray emerging directions are different from one another by a very small angle, two or more different light rays are incident on the pupil of an observer at the position of observation.
In other words, according to the structure, the light sources are provided corresponding to the number of viewpoints for observing a two-dimensional image, and the number of the light sources can be increased to sufficiently reduce the distance between adjacent focal points for reproducing the two-dimensional image, thereby allowing a plurality of parallax images to be incident on a single eye of the observer. It is thus possible to display a stereoscopic image of the “super-multiview regions” in which a plurality of parallax image light rays are incident on the single eye of the observer, and the focal point adjusted in the eyes of the observer is brought near the stereoscopic image, so that fatigue or irritation of the observer can be reduced.
Japanese Patent Application Laid-Open No. H11-103474 (No. 1999-103474) discloses a stereoscopic image display apparatus based on the similar principle. FIG. 54 is a conceptual view of the stereoscopic image display apparatus proposed in Japanese Patent Application Laid-Open No. H11-103474 (No. 1999-103474). In FIG. 54, reference numeral 111 shows a modulation signal generator, 112 a start sensor which detects a start timing of beam scanning, 113 a semiconductor laser driving circuit, 114 a motor control circuit, 115 a semiconductor laser (light source), 116 a collimator lens, 117 a polygon mirror for laser beam scanning in a main scanning direction, 118 an fθ lens, 119 a first cylindrical lens array which deflects a laser beam in the main scanning direction, 121 a galvano mirror for laser beam scanning in a sub-scanning direction, 120 a motor which rotates the galvano mirror 121, 122 a second cylindrical lens array which diffuses a laser beam in a vertical direction, and 123 an observer.
The basic structure of the stereoscopic image display apparatus is the same as that of the stereoscopic image display apparatus shown in FIG. 52. However, the former differs from the latter in that the single laser light source 115 is used and the cylindrical lens array 119 with a periodic structure in the main scanning direction is arranged near the surface on which two-dimensional image information should be formed by laser beam scanning.
In such a structure, the emerging direction of the laser beam from the cylindrical lens array 119 is changed depending on the position of the incidence of the beam. Thereby, beam deflection is repetitively performed a number of times only by laser beam scanning on the cylindrical lens array 119 in the main scanning direction.
FIG. 55 shows how the deflection is achieved. A laser beam a perpendicular to the cylindrical lens array 119 is incident on the array 119 at a position at a large field angle, so that the laser beam is deflected in a direction designated with a′ after it passes through the focal point of the cylindrical lens. After a very short time period, as a laser beam b is incident on the cylindrical lens at a position shown in FIG. 55, the beam is deflected by a smaller deflection angle and emerges in a direction designated with b′. Similarly, laser beams c and d are deflected in directions designated with C′ and d′ in FIG. 55, respectively.
As described above, while the bright points (pixels) are formed corresponding to the number of the light sources in the stereoscopic image display apparatus shown in FIG. 52, the same number of bright points (pixels) are formed by the scanning with the single laser beam in the stereoscopic image display apparatus shown in FIG. 54.
Specifically, as the laser beam is incident on the cylindrical lens array 119, the beam is focused for each of the element lenses constituting the cylindrical lens array 119. Thus, the scanning with the laser beam once on the cylindrical lens array 119 can reproduce a two-dimensional image associated with viewpoints of which the number is equal to the number of the element lenses. This eliminates the need to provide light sources and associated driving circuits or the like corresponding to the number of the viewpoints to enable display of a number of parallax images required for stereoscopic display of the “super-multiview regions” with a simple structure.
The apparatus proposed in Japanese Patent Application Laid-Open No. 11-103474 (No. 1999-103474), however, has the following disadvantages.
As shown in FIG. 54, when the single light source is used to reproduce two-dimensional images associated with the plurality of viewpoints, the number of the reproduced two-dimensional images is the same as the number of the element lenses constituting the cylindrical lens array 119. However, if the number of the element lenses constituting the cylindrical lens array 119 is increased to increase the number of two-dimensional images, each element lens gets smaller and has an insufficient size with respect to the diameter of the laser beam used.
FIG. 56 shows an element lens 119a which has an insufficient size for the diameter of the laser beam. In the case shown in FIG. 56, the laser beam diameter is increased after it passes through the element lens. Each laser beam represents a pixel of the reproduced tow-dimensional image. However laser beam with a diameter larger than a pupil of an eyeball of an observer cannot represents a pixel independent of other laser beams to the observer. Then stereoscopic display in “super-multiview regions”, in which a plurality of beams representing pixels must incident into an eyeball of an observer independently of each other, cannot be achieved.